Typically, metal foams have been mainly used as a light structural material, but recently, open-type light structural bodies having periodic truss structure are being developed as materials replacing such metal foams. Such open-type light structural bodies are configured from truss structures which are designed to have optimal strength and stiffness through an accurate mathematical/mechanical calculation, and thus have superior mechanical properties.
As such a truss structure, an octet truss (R. Buckminster Fuller, 1961, U.S. Pat. No. 2,986,241) having a shape in which regular tetrahedrons and regular octahedrons are combined is the most common. The octet truss is superior in strength and stiffness because constituents of the truss each form regular triangles with each other.
Also, recently, a Kagome truss structure which modifies the octet truss is known (S. Hyun, A. M. Karlsson, S. Torquato, A. G. Evans, 2003, Int. J. of Solids and Structures, Vol. 40, pp. 6989˜6998).
In this case, a truss is configured from long thin members having the same cross-sectional area. When all the constituent members of the truss have the same length, the lengths of the truss elements configuring the Kagome truss are merely a half of those of the truss elements configuring the octet truss, and thus buckling which is a main cause of fracture of the truss may be more effectively prevented, and even when buckling occurs, a collapsing process of the truss is much stable. For reference, FIG. 1 illustrates such a three-dimensional Kagome truss structure.
Also, as methods for manufacturing a truss-like porous lightweight structure, following methods are known.
For example, a manufacturing method (S. Chiras, D. R. Mumm, N. Wicks, A. G. Evans, J. W. Hutchinson, K. Dharmasena, H. N. G. Wadley, S. Fichter, 2002, International Journal of Solids and Structures, Vol. 39, pp. 4093˜4115) in which a truss structure is formed of a resin, and then a metal is cast using the truss structure as a mold. This method requires high costs due to a complex manufacturing process, and is capable of manufacturing only in case of metals having superior castability, and therefore, the application scope thereof is narrow and the resultant thereof is likely to have many defects in cast structure characteristics and lack in strength.
As another example, a method (D. J. Sypeck and H. N. G. Wadley, 2002, Advanced Engineering Materials, Vol. 4, pp. 759˜764), in which holes are periodically formed on a thin metal plate to make the plate in a net shape, a truss intermediate layer is then formed by bending the net-shaped plate, and then face plates are respectively attached to upper and lower portions of the layer, is known. In this method, when wanting to make a multilayered structure having two or more layers, a method, in which the truss intermediate layer made by bending as described above is attached on an upper face plate, and then another face plate is attached again on the face plate, is used. This method has limitations in bonding costs and strength because much material loss is caused during forming holes in the thin metal plate and the number of bonding portions excessively increase when the truss intermediate layer is formed in a multilayer.
As still another example, a method (D. J. Sypeck and H. G. N. Wadley, 2001, J. Mater. Res., Vol. 16, pp. 890˜897), in which a net-like mesh is woven by two wires having directions perpendicular to each other and then the mesh is laminated and bonded. This method also has limitations in bonding costs and strength since the mechanical strength of the truss is decreased because the truss basically does not have an ideal structure such as a regular tetrahedron or a pyramid and since the number of bonding portions is excessively increased because nets are laminated to be bonded to each other.
As an example in which the limitations of the above-described prior arts are addressed, Korean Patent No. 0708483 discloses a method for manufacturing a three-dimensional porous lightweight structure having a form similar to an ideal Kagome or octet truss by making continuous wire groups cross each other in six directions, the wire groups having azimuth angle of approximately 60 degrees or 120 degrees in a space (See FIG. 2), and Korean Patent No. 1029183 discloses a method for manufacturing a three-dimensional porous lightweight structure, as a method capable of more effectively manufacturing such three-dimensional lightweight porous structure, in which a continuous wire is previously formed in a spiral shape, and then the formed spiral wire is inserted into a plurality of woven body spaced apart a predetermined interval from each other while being rotated.
Also, Korean Patent No. 0944326 discloses a method for manufacturing a structure having a similar form to a three-dimensional Kagome truss by using flexible liner bodies, and Korean Patent No. 1114153 discloses a method capable of weaving a structure having a similar form to the three-dimensional Kagome truss which is configured from the above-mentioned flexible liner bodies or stiff spiral wires.
The above-mentioned Korean Patent No. 0708483, Korean Patent No. 1029183, Korean Patent No. 0944326, and Korean Patent No. 1114153 have something in common in that all disclose a method for manufacturing a three-dimensional porous lightweight structure by inserting flexible wires and spiral wires in three out-of-plane directions in a state in which objects similar to a two-dimensional Kagome truss are made in advance and are disposed at regular intervals.
FIG. 3 illustrates a perspective view and a plan view of a three-dimensional lattice truss structure similar to a three-dimensional Kagome truss structure woven by such a method, and FIG. 4 illustrates a unit cell of the structure of FIG. 3.
Referring to FIG. 4, there is a problem in that it is practically difficult to simultaneously cross and assemble in-plane wires 1, 2 and 6 in three directions and out-of-plane wires 3, 4 and 5 in the three directions, and it is difficult to realize a three-dimensional lattice truss structure through a continuous process because there is a limitation in that an object similar to a two-dimensional Kagome truss should be formed in a plane, that is in an xy plane. Also, when a three-dimensional porous lightweight structure having a rectangular parallelepiped shape is manufactured through such a method, there is a problem in that the appearance of the structure deteriorates, and the mechanical strength of the structure also deteriorates because the shape of the periphery of the structure is not uniform for each layer as illustrated in FIG. 3.